Wear-resistant coatings containing hard-phase particles embedded in a more ductile matrix can be produced by several methods. In flame or plasma spray type processes, hard particles such as carbides, borides, or silicides, coated or mixed with more ductile materials such as cobalt, nickel or iron alloys, are heated and propelled toward a substrate by a flame or plasma. Coatings made by these processes commonly contain extensive porosity and are frequently not tightly bonded to the substrate. The hard particles and the ductile matrix material are both subjected to the same thermal environment, with the result that in some cases the hard particles dissolve excessively in the matrix and lose their effectiveness. In detonation-gun processes, powders similar to those used of plasma spraying are employed, but they are propelled by detonating a mixture of acetylene and oxygen. The coatings have better bonding and less porosity than those produced by plasma spraying, but they still suffer from the same defects. The process is expensive and extremely noisy. In weld-cladding a rod of coating material is used to lay down a surface layer by a welding process. This method is applicable only to certain alloys in which the hard and ductile phases are in thermodynamic equilibrium, and is only useful where quite thick layers are desired. The laser spray process employs the laser beam to deliberately heat the coating material to temperatures so high that volatization of the powder will propel it toward the substrate and cause it to be deposited on the substrate. This process is only suitable for the deposition of coating materials with a suitably high vapor pressure at the temperature they achieve in the beam. This high vapor pressure is generally achieved at temperatures above the melting point of the coating material.
It is known in the art to use laser beams in surface alloying for achieving case hardening. For example, see U.S. Pat. No. 4,157,923, which discloses several embodiments for processing selected surface areas such as for surface wear characteristics or extended fatigue life. It is further known as disclosed in U.S. Pat. No. 4,125,926 to build up an aluminum or aluminum alloy surface, such as a piston ring groove, by applying a powder of metal harder than the aluminum and melting it by electromagnetic energy so as to provide a diffusion zone of an alloy of the metal powder and aluminum or aluminum alloy at the interface for establishing a harder surface.
One method of alloying a wear resistant coating to the surface of a substrate is disclosed in United States Defensive Publication No. T967,009. A coating of wear resistant particles and another element, having a lower melting point, are first deposited on the surface to be coated. Thereafter, a laser beam is passed over the deposit to melt the element having the lower melting point and also melt a limited portion of the substrate, thus adhering them together while retaining the wear-resistant particles after solidification of the melt for presenting a wear-resistant surface.